1. Field of the Invention
The present invention relates to a thermal head having an improved printlag efficiency, electronic equipments with such a thermal head, such as printers, word processors, facsimile machines and plotters, and a process of making such a thermal head.
2. Description of the Related Art
The thermal heads can be classified into partial-glaze type, double-partial-glaze type and true-edge type. As shown in FIG. 1, the partial-glaze type thermal head comprises an insulation substrate 11, a partial glaze layer 12 formed on the substrate adjacent to its edge portion, having a width equal to about 300-1200 microns and an outwardly convex configuration, a resistive film layer 13 formed over the partial glaze layer 12, common and discrete electrodes 15 and 16 formed on the resistive film layer 13 at the top positions of the glaze layer 12 opposite to each other to form a heating section 14 on the top of the glaze layer 12 and a protective film 17 covering these layers as a whole. The double-partial-glaze type thermal head is one similar to the partial-glaze type thermal head except that a portion of the glaze layer 12 placed at the heating section 14 is formed into an upwardly convex configuration by glaze etching or the like, as shown in FIG. 2.
The true-edge type thermal head is one where in the glaze layer 12 and the heating section 14 are formed to cover the edge of the insulation substrate 11, as shown in FIG. 3. FIG. 4 shows a modification of the thermal head shown in FIG. 3, in which the edge portion of the insulation substrate 11 is slantingly cut to provide a slope 11b adjoining the top face 11a of the insulation substrate 11 and an edge face 11c adjoining the slope 11b and extending perpendicular to the top face 11a. Glaze layers 12a, 12b and 12c are formed over the respective races 11a, 11b and 11c. The heating section 14 is formed at the slope 11b.
As shown in FIG. 5, the thermal transfer is carried out against an ink ribbon 18 and a sheet to be printed on 19 which are held between the glaze layer 12 and a rubber platen 20, with tile ink ribbon 18 being urged to the sheet to be transferred 19 by the glaze layer 12. Since tile top of the glaze layer 12 exerting the maximum urging force to the ink ribbon 18 includes the heating section 14, the heating and urging of the ink ribbon 18 will be simultaneously made on the thermal transfer. In order to print a rough sheet and to improve the printing efficiency, the thermal head requires that a heating resistor portion concentrates its pressure onto tile ink ribbon, the sheet to be printed on and the platen. For example, if a rough sheet is to be printed, a letter pattern is thermally cut away from the ink ribbon 18. The cut letter pattern is then transferred to the sheet to be printed 19 under tile urging force from the glaze layer 12. In the partial-glaze and double-partial-glaze type systems, however, an area at which the glaze layer 12 having the heating section 14 engages the rubber platen 20 is relatively large. This raises a problem in that the pressure from the heating section 14 is not sufficiently concentrated onto the ink ribbon 18. Although the true-edge type system (FIG. 3) has been developed to overcome such a problem, it cannot presently provide a sufficient advantage since the substrate has an undesirable thickness equal to about 2 mm. A common problem with the aforementioned systems of the prior art is that when a substrate is to be worked at its side face to print a plurality of thermal heads thereon, they cannot be formed at the same time. As a result, the manufacturing cost per thermal head will be increased.